The invention deals with the rotationally true clamping of rotating machine elements, in particular in a balancing machine.
Modern material-removing machine tools often use high-speed drills, mills or grinders (with rotational speeds in the range from 10,000 to 20,000 revolutions per minute, sometimes even up to 40,000 revolutions per minute or more), the cutting capacity of which is based not so much on a high cutting force and cut depth but rather on the high rotational speeds. On account of the high centrifugal forces which are generated at such rotational speeds, even a slight unbalance of the tools and their holders may have pronounced negative consequences. Firstly, the spindle bearings in which the drive spindle of the machine tool is mounted are exposed to nonuniform loads and become worn more quickly. Secondly, the cutting result deteriorates, which may become obvious, by way of example, through an increased surface roughness of the cut surface. Where the present text speaks of tool holders, this term is understood as meaning all possible holding means in which the actual milling, drilling or grinding tools can be clamped and which in turn can be coupled to the drive spindle of the machine tool. The term is aimed in particular at commercially available tool holders with standardized steep-taper or hollow steep-taper coupling shanks.
For the above reasons, the tool holders are first balanced on a balancing machine before being used in the machine tool. Conventional measures for balancing are fitting additional weights or introducing weight-reducing bores. To allow a measured inbalance to be unambiguously ascribed to the tool holder, extremely high demands are imposed on the unbalance freedom of the rotating parts of the balancing machine. This is because if the measured unbalance were to emanate at least in part from the balancing machine itself, the balancing measures taken on the basis of the results of the measurement on the tool holder could in the worst possible scenario even increase the unbalance of the tool holder. The clamping of the tool holder in the balancing machine has proven to be a problem area. A clamping device which is provided in a known balancing machine for clamping the tool holder comprises a base body unit which is attached to a machine spindle which is driven about an axis of rotation and a receiving opening which is central with respect to the axis of rotation and into which a coupling shank of the tool holder can be inserted axially from one end. The receiving opening passes all the way through the base body unit in the axial direction. A chuck is arranged in the receiving opening, which chuck engages on the coupling shank of the tool holder and can be actuated by a chuck-actuating unit which is guided on the base body unit in such a manner that it can be displaced in the axial direction relative to the latter. The chuck-actuating unit has double guidance on the base body unit from two pairs of sliding surfaces. To achieve highly accurate guiding, these pairs may be designed as transition fits, but this means that the chuck-actuating unit can only be displaced with very great difficulty with respect to the base body unit. Greater tolerances allow easier adjustment of the chuck-actuating unit. However, it has been found that these tolerances led to true-running faults making their presence felt in the measurement results, which in view of the high demands imposed on the true-running accuracy of the balancing machine, considerably impair the reproducibility of the measurements. This effect is intensified further by the double guidance of the chuck-actuating unit on the base body unit. However, reproducible unbalance measurements are necessary for high-precision fine balancing of the tool holder.
Accordingly, it is an object of the invention to provide a clamping device which is easy to operate, in particular for a balancing machine, with a high level of true-running accuracy.
In achieving this object, the invention is based on a clamping device for clamping a machine element, in particular a tool holder for a drilling, milling or grinding tool or the like, which rotates about an axis of rotation, comprising a base body unit which has a receiving opening which is central with respect to the axis of rotation and into which a coupling shank of the machine element can be fitted axially from one end, a chuck which is arranged in the receiving opening, and a chuck-actuating unit, which is guided on the base body unit in such a manner that it can be displaced in the axial direction relative to the latter, for actuating the chuck.
According to the invention, it is provided that the chuck-actuating unit is axially guided by means of a rolling body arrangement on the base body unit, the rolling bodies of which arrangement roll along rolling surfaces of the base body unit and of the chuck-actuating unit. The rolling body arrangement allows easy yet precise axial guidance of the chuck-actuating unit, in particular if the rolling bodies are fitted with prestress between the base body unit and the chuck-actuating unit. Tolerances which may impair the reproducibility of the measurements only have to be accepted to a significantly lesser extent, if at all, than is the case with the sliding fits of the known solution. Moreover, the guidance of the chuck-actuating unit by rolling bodies is distinguished by a reduced cost.
Although in principle it is conceivable to use rolling bodies designed in other ways, it will be expedient to use balls which are held in a cage inserted between the base body unit and the chuck-actuating unit. Ball cages of this type are commercially available at low cost. In order to keep the weight of the components of the clamping device which move relative to the base body unit low, the cage is expediently made from plastic. A small number of balls are still sufficient to achieve highly accurate guiding if the balls are held individually, offset from one another in the circumferential direction, in each case in one recess in the cage.
A compact structure of the clamping device results from the fact that the rolling body arrangement and the chuck are arranged at least partially inside one another in the axial direction. At the same time, it is in this way possible to use a chuck with a relatively great axial length and with a correspondingly high flexibility of its tongues without the overall space taken up being subject to restrictions from the rolling body arrangement.
The rolling body arrangement may be accommodated radially between a cylindrical inner lateral surface section of the receiving opening and a cylindrical outer lateral surface section of a shank part, which extends axially in the receiving opening, of the chuck-actuating unit. In this case, it is expedient for smooth running if the shank part is formed integrally with a chuck-engagement head, which engages on the chuck, of the chuck-actuating unit.
It is possible to provide for the shank part to extend out of an end of the receiving opening which is remote from the machine element, for, at that location, a force-introducing body, which serves to introduce the actuating force, to adjoin the shank part, and for this force-introducing body to be approximately in the form of a disk.
In a variant of the known solution discussed in the introduction, the chuck is held on the base body unit and is supported, by means of a clamping section, on the outer lateral surface of a cone head, which is substantially enclosed by this unit and tapers away from the machine element, of the chuck-actuating unit. In this arrangement, the chuck engages, by means of a radially projecting holding flange in the region of its end which is remote from the machine element, in an annular recess which has been machined from the receiving opening radially into the base body unit, where it is held with clearance to move.
By contrast, in the clamping device according to the invention, it is preferably provided that the chuck, in the region of its end which is axially remote from the machine element, is mounted fixedly on the base body unit. The fixed mounting of the chuck on the base body unit has led to an improvement of the true-running accuracy of the clamping device irrespective of the guidance of the chuck-actuating unit by rolling bodies.
In order, if desired, to be able to exchange the chuck, the chuck will expediently be screwed to the base body unit and, for this purpose, will have a plurality of axial receiving holes, which are distributed in the circumferential direction, for receiving attachment screws. However, the possibility of a press-fit connection between the chuck and the base body unit should not be excluded. A large axial space for the chuck can be provided by the fact that the chuck extends as far as the bottom of an annular recess, which is central with respect to the axis of rotation and is machined into the base body unit from that side of this unit which axially faces the machine element, and is secured in this annular recess.
In the abovementioned variant of the known clamping device, the base body unit comprises a first partial base body, with bearing surfaces which are intended to be in contact with the coupling shank of the machine element, and a second partial base body, which is produced separately from the first partial base body, is connected firmly but releasably to this first partial base body and on which the chuck-actuating unit is axially guided. In this arrangement, the chuck is held on the first partial base body.
In the clamping device according to the invention, however, the chuck is preferably held on the second partial base body. The two-part structure of the base body unit allows a modular system to be achieved. The bearing surfaces of the first partial base body can, within this modular system, be optimally adapted to the machine element which is in each case to be clamped. By way of example, a set of suitably different first partial base bodies can be manufactured for a set of steep-taper tool holders of different sizes. All these first partial base bodies have a uniform interface for interaction with the second partial base body, for which reason a single second partial base body, which can be combined with any of the first partial base bodies, is sufficient. The chuck generally also requires a different shape and/or size depending on the machine element which is in each case to be clamped. If, as in the known solution, the chuck is held on the first partial base body, it may be necessary, on account of the varying configuration of the first partial base body, for the interface between the first partial base body and the chuck also to be designed differently depending on the particular machine element. By contrast, if the chuck is held on the second partial base body, it is possible for all chucks which are manufactured for the different machine elements to be designed with a uniform interface for interaction with the second partial base body, which overall is of benefit to the design and manufacturing outlay on the modular system.
In order, when measuring the unbalance of the machine element, to be able to determine the precise angular location of an unbalance, an angle reference can be obtained by the fact that the base body unit bears at least one indexing projection, which is intended to engage in an indexing cutout in the coupling shank of the machine element. Naturally, the indexing projection may, if desired, be releasable if machine elements which are designed without a suitable indexing cutout are being clamped.
The possibility of arranging the rolling body arrangement in the receiving opening around a shank part of the chuck-actuating unit has previously been described.
A different location for fitting the rolling body arrangement may consist in the fact that the chuck-actuating unit, on that side of the base body unit which is axially remote from the machine element, has a force-introducing body, which serves to introduce an actuating force and has an annular flange, which projects toward the machine element and encloses an axial end section of the base body unit, and that the rolling body arrangement is accommodated radially between a cylindrical inner lateral surface section of the annular flange and a cylindrical outer lateral surface section of the axial end section of the base body unit. To avoid double guidance of the chuck-actuating unit, the latter is preferably unguided in the axial direction within the receiving opening.
The above text has discussed a variant in which the chuck is held on the base body unit. The chuck may alternatively be held on the chuck-actuating unit, so that it can be moved axially in the receiving opening. To clamp the machine element, the chuck-actuating unit then exerts a pulling action on the chuck. In this design, it is preferable for the chuck to be held radially in a clearance fit on the chuck-actuating unit and to be guided axially on the inner lateral surface of the receiving opening. The clearance fit leads to the chuck being decoupled from the chuck-actuating unit; instead, the chuck is guided separately from the chuck-actuating unit on the base body unit, namely on the inner lateral surface of the receiving opening. It has been found that the true-running accuracy of the clamping device is improved by this measure, even irrespective of the rolling body guidance.
Preferably, the chuck, in the region of its end which is axially remote from the machine element, has a cylindrical outer lateral surface section, by means of which it is axially guided on a cylindrical inner lateral surface section of the receiving opening. The cylindrical fit leads to the chuck being guided on the base body unit at its end which is remote from the machine element, so that true-running errors which could arise from any tensile expansion of the chuck are avoided. In addition, the chuck may be axially guided on the inner lateral surface of the receiving opening in the region of its end which is close to the machine element. Guiding the chuck at both ends in this way makes it possible to achieve an extremely high true-running accuracy.
To decouple the chuck from the chuck-actuating unit, it is also possible for the chuck to have an axial clearance with respect to the chuck-actuating unit. To prevent true-running errors caused by the positioning of chuck and chuck-actuating unit with respect to one another not being optimum from arising when the clamping device is rotating, it is possible for an engagement surface of the chuck, which is under tensile load from the chuck-actuating unit, to be designed as a centering surface, which is inclined toward the axis of rotation, for the chuck-actuating unit. The centering of the chuck-actuating unit on the chuck, which for its part is guided in the receiving opening, ensures extremely smooth running.
The chuck is preferably of single-part design and has a plurality of resilient tongues which are distributed in the circumferential direction and are separated from one another by axially extending slots which penetrate radially through the chuck and are open toward the machine element and closed away from the machine element.
At various points of the above text, mention has been made of a force-introducing body, which is arranged on that side of the base body unit which is remote from the machine element, as part of the chuck-actuating unit. This force-introducing body preferably forms a first support for spring means, for example a plurality of helical compression springs which are distributed in the circumferential direction and preload the chuck-actuating unit into a clamping position, in which the machine element is clamped fixedly by the chuck. The base body unit, in particular, serves as a second support for the spring means. In a preferred configuration of the clamping device according to the invention, the force-introducing body at the same time serves as the piston of a piston-cylinder arrangement, by means of which the chuck-actuating unit and therefore the chuck can be transferred into a release position, in which the machine element can be removed from the clamping device. The cylinder of this piston-cylinder arrangement may be formed by a drive spindle, which is designed as a hollow spindle and on which the base body unit can be mounted, expediently by means of screws. In order, in particular with such a design of the clamping device, to reduce the weight of the components which can move in the axial direction with respect to the base body unit, it is possible for at least one cutout, which serves primarily to save weight, to be machined into the force-introducing body or piston. A low weight of these axially moveable components has proven expedient in order to achieve highly smooth running of the clamping device.
It will be understood that the clamping device explained above is also suitable for clamping tools or workpieces which are to be machined in a machine tool, although a preferred field of application is the clamping of tool holders which are to be balanced in a balancing machine.
According to a further aspect, the invention relates to a clamping device for clamping a machine element, in particular a tool holder for a drilling, milling or grinding tool or the like, which rotates about an axis of rotation, comprising a drive spindle which is mounted so that it can rotate about the axis of rotation, a clamping set which can be mounted on the spindle, is preloaded in a clamping position by spring means, receives the machine element and clamps it in a rotationally fixed manner in its clamping position, a release device which, in order to release the machine element, transfers the clamping set out of its clamping position into a release position and has a coupling member, which is arranged on a stationary part of the clamping device and, in the at-rest state of the clamping device, links the release device to components of the clamping device which rotate when the clamping device is rotating.
According to the invention, in a clamping device of this type it is provided that the coupling member can be lifted off the rotating components of the clamping device, in such a manner that when the clamping device is rotating the coupling member is decoupled from the rotating components of the clamping device.
This measure acts to achieve the object which was set at the outset to the extent that, as a result of the coupling member being decoupled from the clamping device, the smooth running of the device in rotation is not disrupted by the coupling member and any frictional effects which arise when there is a constant coupling between the coupling member and the clamping device and would impair the ease of movement of the clamping device, are avoided.
Preferably, the coupling member can be displaced between an advanced position, in which it is coupled to the rotating components, and a retracted position, in which it is decoupled from the rotating components, and is resiliently preloaded into its retracted position. The preloading of the coupling member ensures a high level of operational reliability, since consequently the coupling member, during rotation of the clamping device, cannot unintentionally come into contact with the rotating components of the clamping device.
Preferably, the coupling member can be transferred out of its retracted position into its advanced position by fluid pressure, in particular pneumatic pressure, although the possibility of electromagnetic actuation of the coupling member, for example by means of a solenoid, is not ruled out.
In order to be able to apply the generally high release forces which the clamping set requires for release, it is recommended that the clamping set can be transferred out of its clamping position into its release position by fluid pressure, in particular pneumatic pressure, and for this purpose a piston-cylinder arrangement, the piston of which transmits a release force to the clamping set, is provided in the force-transmission path of the release force, which is generated by the release device. Since the spindle is often designed as a hollow spindle into which the clamping set is fitted from an open axial end, it may expediently at the same time form the cylinder of the piston-cylinder arrangement.
According to a preferred embodiment, the piston is fixedly connected to an axially moveable actuating member of the clamping set, a pressure chamber formed in the spindle is delimited on one side in the axial direction by the piston and on the other side in the axial direction by a closure wall which is arranged fixedly on the spindle, the closure wall has a connection opening for connection of a pressurized-fluid feedline, and the pressurized-fluid feedline has a connection stub which can be introduced into the connection opening and which forms the coupling member. This embodiment is advantageous since the only measure which is required in order to be able to introduce pressurized fluid into the pressure chamber and thereby release the machine element is for the connection stub to be introduced into the connection opening.
To prevent contamination from penetrating into the pressurized-fluid feedline, the connection stub may, in its retracted position, form a blocking location in the pressurized-fluid feedline, which is opened when the connection stub is transferred into its advanced position.
The structural outlay for the adjustment mechanism which allows the adjustment of the connection stub can be kept at a particularly low level if the connection stub can be transferred into its advanced position by the fluid pressure of a pressurized-fluid supply, which at the same time feeds pressurized fluid into the pressurized-fluid feedline.
In order to keep the outlay on components and design for the clamping set at a low level, it is recommended for the piston at the same time to be used to support the spring means.
When changing the machine element which is to be clamped, it may be necessary for the clamping set to be removed from the spindle in order for it to be changed over in a suitable way. If, during removal of the clamping set, the operator has forgotten to reduce the pressure in the pressure chamber beforehand, for example because he has forgotten to retract the connection stub, there is a risk that the clamping set will be suddenly thrown off the spindle by the pressure which is prevailing in the pressure chamber as soon as its attachment screws or other attachment members by means of which it is attached to the spindle are released. To prevent this risk, it is possible for a relief channel, which leads out of the pressure chamber at one end, and at the other end opens out in a spindle-side bearing surface for the clamping set, in particular an axial end face of the spindle, to be formed in the circumferential wall of the spindle.
According to a further aspect, the invention relates to a clamping device for clamping a machine element, in particular a tool holder for a drilling, milling or grinding tool or the like, which rotates about an axis of rotation, comprising a drive spindle which is mounted so that it can rotate about the axis of rotation and is designed as a hollow spindle, and a clamping set which can be mounted on the spindle, clamps the machine element in a rotationally fixed manner in a clamping position, and comprises a base body unit which can be inserted into the spindle from one spindle end, can be connected to the spindle in a rotationally and axially fixed manner and has a receiving opening which is central with respect to the axis of rotation and into which a coupling shank of the machine element can be fitted in the axial direction. According to the invention, in this arrangement it is provided that a ball cage, which centers the base body unit on the spindle, is fitted radially between the base body unit and the spindle.
The ball cage allows highly accurate centering of the base body unit on the spindle, which is of benefit to the true-running accuracy. Moreover, the ball cage allows simple insertion of the base body unit into the spindle and removal of this unit from the spindle, so that this aspect of the invention also serves to achieve the object which was set out at the outset.
Furthermore, the invention relates to a measuring device for measuring the unbalance of a machine element, in particular a tool holder for a drilling, milling or grinding tool or the like, which rotates about an axis of rotation, comprising a drive spindle which is mounted in a spindle housing in such a manner that it can rotate about the axis of rotation, a frame, from which the spindle housing is suspended, a force sensor arrangement which detects unbalance forces on the spindle housing, and clamping means for clamping the machine element in a rotationally fixed manner on the spindle. In this arrangement, the clamping means may be formed by a clamping device according to one of the aspects which have been explained above.
In rotary operation, lack of roundness of the spindle bearings and micro-deformation of the spindle housing may cause measurement errors which distort the measurement results. It has been found that measurement errors of this type can be minimized if the spindle housing is suspended from the frame by means of a leaf spring arrangement in such a manner that it is fixed so that it cannot be pushed or pulled in a first direction which is normal to the axis, but can be pivoted in a second direction which runs normal to the axis and transversely, in particular substantially perpendicularly, to the first direction, and the force sensor arrangement is arranged so as to detect force components in the second direction. A solution in which the leaf spring arrangement comprises two leaf springs which are arranged at an axial distance from one another, extend radially in the first direction and by means of which the spindle housing is suspended from the frame, has proven particularly expedient.